function: set/array of something impossible?

[Eugene Loza]

It seems like returning a function result as a set/array without making the set a declared type doesn't work? The documentation seems to be a bit cryptic on that "The result type of a function can be any previously declared type".

In other words this doesn't compile:

Code: Pascal  [Select][+][-]

1. function A: set of Byte; // Error: Type identifier expected

neither does this

Code: Pascal  [Select][+][-]

1. function B: array of Byte; // Error: Type identifier expected

While this works:

Code: Pascal  [Select][+][-]

1. type
2.   TSetOfByte = set of Byte;
3. function A: TSetOfByte;
4. type
5.   TArrayOfByte = array of Byte;
6. function B: TArrayOfByte;

Also this works:

Code: Pascal  [Select][+][-]

1. function A: specialize TList<Byte>;

To be more specific about my usecase, I wanted to make a function that does the following:

Code: Pascal  [Select][+][-]

1. generic function StringToEnumsSet<T>(const AString: String): set of T;

I don't think I can forward-declare type set of T, can I?

[MarkMLl]

That sounds like the same rule that the type of a parameter has to be predefined.

MarkMLl

[Warfley]

Yes this is currently not possible with generic functions, you could make a generic type to do a workaround:

Code: Pascal  [Select][+][-]

1. type
2.   generic TStringToEnumSet<T> = record
3.     type TEnumSet = set of T;
4.     class function Convert(const AString: String): TEnumSet;
5.   end;

[Eugene Loza]

Oh, that makes sense! Should fit my usecase. I'll try it Thank you!

[Thaddy]

For the array, simply add types to the uses clause

Code: Pascal  [Select][+][-]

1. uses types;
2. function test:TByteDynArray;

There are many more predeclared dynarrays in types.
Also note that in trunk it is possible to forward declare generic methods/functions/procedures.
https://wiki.freepascal.org/FPC_New_Features_Trunk#Support_for_forward_declarations_of_generic_types

[PascalDragon]

Also this works:

Code: Pascal  [Select][+][-]

1. function A: specialize TList<Byte>;

This is because specializations are considered declared types. That also means that you can use specialize TArray<Byte> for example as result type.

To be more specific about my usecase, I wanted to make a function that does the following:

Code: Pascal  [Select][+][-]

1. generic function StringToEnumsSet<T>(const AString: String): set of T;

I don't think I can forward-declare type set of T, can I?

For sets you currently need to use the workaround that Warfley mentioned.

Also note that in trunk it is possible to forward declare generic methods/functions/procedures.
https://wiki.freepascal.org/FPC_New_Features_Trunk#Support_for_forward_declarations_of_generic_types

Thaddy, please, read and understand things you're posting: The link you mentioned talks about forward declaring generic types and you say generic methods/functions/procedures.
Newsflash: for generic methods forward declaration is not necessary, because they're always forward declared inside a type anyway and generic functions/procedures supported forward declaration since their introduction.

[Eugene Loza]

Got to try the workaround, unfortunately it doesn't seem to work...? Or maybe I'm doing something wrong?

[jamie]

Code: Pascal  [Select][+][-]

1. unit Unit1;
2.  
3. {$mode Delphi}{$H+}
4.  
5. interface
6.  
7. uses
8.   Classes, SysUtils, Forms, Controls, Graphics, Dialogs, StdCtrls,
9.   generics.collections;
10.  
11. type
12.  
13.   { TForm1 }
14.  
15.   TForm1 = class(TForm)
16.     Button1: TButton;
17.     procedure Button1Click(Sender: TObject);
18.   private
19.  
20.   public
21.  
22.   end;
23. Function Test<T>(A:array of T):THashSet<T>;
24. var
25.   Form1: TForm1;
26.  
27. implementation
28.  
29. {$R *.lfm}
30.  
31. Function Test<T>(A:Array of T):THashSet<T>;
32. var
33.   I:T;
34. Begin
35.    result := ThashSet<T>.Create;
36.    For I in A do Result.Add(I);
37. end;
38.  
39. { TForm1 }
40.  
41. procedure TForm1.Button1Click(Sender: TObject);
42. Var
43.   S:THashSet<Byte>;
44. begin
45.   S:=Test<Byte>([34,56,23,89,12]);
46.   Caption := S.Extract(56).ToString; //Returns 0 if not found
47.   S.Free;
48. end;
49.  
50. end.
51.  
52.  

is this what you are looking for? a SET generics.

I used the currently available one in the tools, but I wrote one using  the fgl.
the Generics.Colllections is a  little heavy, but it get you there.